U.S. patent application number 15/748590 was filed with the patent office on 2019-01-03 for method for frequency control of a piezoelectric transformer and circuit arrangement comprising a piezoelectric transformer.
This patent application is currently assigned to EPCOS AG. The applicant listed for this patent is EPCOS AG. Invention is credited to Markus PUFF, Michael WEILGUNI.
Application Number | 20190008027 15/748590 |
Document ID | / |
Family ID | 56551368 |
Filed Date | 2019-01-03 |
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United States Patent
Application |
20190008027 |
Kind Code |
A1 |
WEILGUNI; Michael ; et
al. |
January 3, 2019 |
Method for Frequency Control of a Piezoelectric Transformer and
Circuit Arrangement Comprising a Piezoelectric Transformer
Abstract
A method for frequency control of a piezoelectric transformer
and a circuit arrangement including a piezoelectric transformer are
disclosed. In an embodiment, the method includes exciting a
piezoelectric transformer on an input side with an AC voltage of
predetermined frequency as input voltage, capturing a phase
information for an input impedance of the piezoelectric transformer
in a feedback path, evaluating the captured phase information in
respect of a predetermined phase criterion, and regulating the
frequency of the AC voltage on a basis of the evaluated phase
information.
Inventors: |
WEILGUNI; Michael;
(Hagenberg, AT) ; PUFF; Markus; (Graz,
AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EPCOS AG |
Munchen |
|
DE |
|
|
Assignee: |
EPCOS AG
Munchen
DE
|
Family ID: |
56551368 |
Appl. No.: |
15/748590 |
Filed: |
July 8, 2016 |
PCT Filed: |
July 8, 2016 |
PCT NO: |
PCT/EP2016/066332 |
371 Date: |
January 29, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05H 1/2475 20130101;
H01L 41/044 20130101; H01L 41/107 20130101; H05H 2001/2481
20130101 |
International
Class: |
H05H 1/24 20060101
H05H001/24; H01L 41/107 20060101 H01L041/107; H01L 41/04 20060101
H01L041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 29, 2015 |
DE |
102015112410.6 |
Claims
1-10. (canceled)
11. A method for frequency regulation for a piezoelectric
transformer, the method comprising: exciting a piezoelectric
transformer on an input side with an AC voltage of predetermined
frequency as an input voltage; capturing a phase information for an
input impedance of the piezoelectric transformer in a feedback
path; evaluating the captured phase information with respect of a
predetermined phase criterion; and regulating the frequency of the
AC voltage on based on the evaluated phase information.
12. The method according to claim ii, wherein the predetermined
phase criterion is chosen as a zero or a local extreme of a phase
angle of the input impedance as a function of the frequency of the
AC voltage, and wherein evaluating the captured phase information
comprises a rating of an adequate satisfaction of the predetermined
phase criterion.
13. The method according to claim ii, wherein a phase detector in
the feedback path compares a signal of the input voltage with a
signal proportional to the input current on the input side of the
piezoelectric transformer so that the phase detector generates an
output signal that is proportional to an absolute value of an phase
angle between the input voltage and the input current and serves as
phase information for the input impedance of the piezoelectric
transformer.
14. The method according to claim ii, further comprising sampling a
signal proportional to the input current on the input side of the
piezoelectric transformer, and using a Fourier transformation to
compute an phase angle of the signal from the samples, wherein the
phase angle serves as phase information for the input impedance of
the piezoelectric transformer.
15. The method according to claim 14, wherein sampling the signal
is performed by an impedance analyzer.
16. A method for operating a piezoelectric transformer as a plasma
generator, the method comprising: applying an input voltage at a
frequency regulated in accordance with the method of claim 11, on
the input side; and converting the input voltage to an output high
voltage so that a plasma is produced on an output side on account
of ionization of an operating gas flowing around the plasma
generator.
17. A circuit arrangement comprising: a piezoelectric transformer
having an input side and an output side; an AC voltage source
configured to produce an input voltage of a predetermined frequency
on the input side of the piezoelectric transformer; a detector
located in a feedback path between the input side of the
piezoelectric transformer and the AC voltage source, wherein the AC
voltage source is configured to capture a phase information for an
input impedance of the piezoelectric transformer; and a regulator
configured to evaluate the captured phase information with respect
to a predetermined phase criterion and to prescribe a frequency for
producing the input voltage to the AC voltage source on a basis of
the evaluated phase information.
18. The circuit arrangement according to claim 17, wherein the
detector comprises a phase detector arranged such that an signal of
the input voltage is applied to a first input of the phase detector
and a signal proportional to an input current on the input side of
the piezoelectric transformer is applied to a second input of the
phase detector, and wherein the phase detector is configured to
output an output signal that comprises the phase information of the
input impedance at an output.
19. The circuit arrangement according to claim 18, wherein the
phase detector comprises an XOR gate and a low-pass filter, which
are connected in series, and wherein an arithmetic sign signal for
the input voltage is applied to a first input of the XOR gate and
an arithmetic sign signal proportional to the input current on the
input side of the piezoelectric transformer is applied to a second
input of the XOR gate such that an output signal is producible at
the output of the XOR gate as a comparison signal that can be
averaged by the low-pass filter to produce a signal that is
proportional to the absolute value of an phase angle of the input
impedance of the piezoelectric transformer.
20. The circuit arrangement according to one of claim 19, wherein
the piezoelectric transformer is part of a piezoelectric plasma
generator configured to produce an output high voltage on the
output side from the input voltage on the input side so that a
plasma is produced on the output side on account of ionization of
an operating gas flowing around the plasma generator.
21. The circuit arrangement according to claim 17, wherein the
detector comprises an impedance analyzer arranged such that a
signal proportional to the input current on the input side of the
piezoelectric transformer is applied to the input of the impedance
analyzer.
22. The circuit arrangement according to claim 21, wherein the
impedance analyzer is arranged to sample the signal and to supply
samples to a Fourier transformation to compute an phase angle of
the signal, which corresponds to the phase angle of the input
impedance of the piezoelectric transformer.
23. The circuit arrangement according to claim 17, wherein the
piezoelectric transformer is a piezoelectric plasma generator
configured to produce an output high voltage on the output side
from the input voltage on the input side so that a plasma can be
produced on the output side on account of ionization of an
operating gas flowing around the plasma generator.
Description
[0001] This patent application is a national phase filing under
section 371 of PCT/EP2016/066332, filed Jul. 8, 2016, which claims
the priority of German patent application 10 2015 112 410.6, filed
Jul. 29, 2015, each of which is incorporated herein by reference in
its entirety.
TECHNICAL FIELD
[0002] The invention relates to a method for frequency regulation
for a piezoelectric transformer and to a circuit arrangement,
comprising a piezoelectric transformer and an AC voltage source for
producing an input voltage on an input side of the piezoelectric
transformer.
BACKGROUND
[0003] Piezoelectric transformers allow the conversion of a
supplied AC voltage as input voltage on an input side into a higher
or lower AC voltage as output voltage on an output side of a
piezoelectric element. The piezoelectric element is frequently
constructed from a lead zirconate titanate compound (PZT). On the
basis of an appropriate polarization of the input and output sides
of the piezoelectric element, the latter is deformed, e.g., in the
thickness direction when a sinusoidal AC voltage is applied on the
input side on account of the inverse piezoelectric effect, as a
result of which an oscillation is produced in the longitudinal
direction of the piezoelectric element. This in turn produces a
corresponding output voltage on the output side on account of the
direct piezoelectric effect.
[0004] If the frequency of the applied input voltage matches the
resonant frequency of the piezoelectric element, this results in an
electromechanical resonance of the element, which means that the
mechanical vibration reaches a maximum. In this manner, a very high
output voltage can be produced on the output side of the
piezoelectric element. One application involves, by way of example,
operating a piezoelectric transformer as a plasma generator,
wherein ionization of an operating gas flowing around the plasma
generator takes place on account of a high output voltage on the
output side of the plasma generator, so that a plasma is
produced.
[0005] When operating a piezoelectric transformer, it is further
always desirable to operate the transformer at a maximum
efficiency. The maximum efficiency can only ever be achieved at one
particular frequency. This frequency is dependent on numerous
parameters, inter alia on the input voltage and the operating
environment used. Finding the maximum efficiency therefore requires
information from the component.
[0006] There are already multiple possibilities for the frequency
regulation of conventional piezoelectric transformers. For example,
the voltage on the secondary side (output voltage) can be
considered. Another possibility is the use of an additional
electrode on the transformer to obtain a feedback signal.
[0007] A disadvantage in the case of the first possibility is that
every instance of the output voltage being tapped off influences
the amplitude of the output voltage and hence the oscillatory
response of the piezo element and finally the manner of operation
of the piezoelectric transformer. Particularly when the
piezoelectric transformer is used as a plasma generator, such a
possibility for the frequency regulation would have a greatly
adverse influence on the manner of operation of the plasma
generator.
[0008] By contrast, the second variant has the disadvantage that it
necessitates further contact connection of the piezoelectric
component, which complicates the design.
SUMMARY OF THE INVENTION
[0009] Embodiments provide regulating a piezoelectric transformer,
e.g., a piezoelectric plasma generator, in respect of the frequency
such that it is (always) operated at maximum efficiency and the
manner of operation is nevertheless influenced as little as
possible given a simple design.
[0010] In various embodiments the method comprises the steps of:
exciting a piezoelectric transformer on an input side with an AC
voltage of predetermined frequency as input voltage, capturing a
phase information for the input impedance of the piezoelectric
transformer in a feedback path, evaluating the captured phase
information in respect of a predetermined phase criterion, and
regulating the frequency of the AC voltage on the basis of the
evaluated phase information.
[0011] In various further embodiments, a phase information, that is
to say an information about the phase angle between sinusoidal
input voltage and sinusoidal input current, which corresponds to
the phase angle of the input impedance, is detected. Finally, the
frequency of the AC voltage can be regulated on the basis of the
evaluated phase information.
[0012] The advantage of such a method is that, for a frequency
regulation, only an information captured on the input side of the
piezoelectric transformer is used as a criterion for the frequency
regulation. In this manner, the operating response of the
piezoelectric transformer is barely or only insignificantly
influenced in practice. In particular, the tapping-off of signal
information on the output side of the piezoelectric transformer is
dispensed with. Nevertheless, the piezoelectric transformer can
always be operated under optimum conditions solely on the basis of
the information captured on the input side. A crucial advantage in
comparison with conventional concepts is the high efficiency that
can be attained in the process.
[0013] A principle of the method is that, solely on the basis of
the phase or the phase angle of the input impedance of the
piezoelectric transformer, it is possible to operate the
transformer at maximum efficiency each time under arbitrary
external conditions. This means that solely on the basis of an
evaluation of the captured phase information in respect of a
predetermined phase criterion, it is possible for a particular
operating frequency at which the predetermined phase criterion is
satisfied and a maximum efficiency exists to be inferred
algorithmically. Therefore, the piezoelectric transformer can be
regulated to an operating frequency of the AC voltage, so that the
transformer operates at a maximum efficiency at this frequency.
[0014] The method may involve the predetermined phase criterion
advantageously being chosen as one or more zeros or a local extreme
of the phase angle of the input impedance as a function of the
frequency of the AC voltage. For example, the local extreme may be
a local minimum of the phase angle of the input impedance. The
evaluating of the captured phase information advantageously
comprises a rating of the adequate satisfaction of the
predetermined phase criterion. The characteristic of the phase
angle of the input impedance as a function of the frequency of the
AC voltage therefore permits conclusions to be drawn about
particular operating frequencies at which an efficiency of the
piezoelectric transformer is optimum under particular operating
conditions.
[0015] Mathematical formulation of one or more of these phase
criteria and appropriate implementation of regulation may allow
continuous regulation of the frequency to be effected such that the
captured phase information is always evaluated to establish whether
the phase criterion/criteria is/are adequately satisfied. It is
therefore possible to regulate to an operating frequency
accompanied by a corresponding phase criterion as the optimum
operating frequency of the piezoelectric transformer with maximum
efficiency. In this manner, it is possible to ascertain an optimum
operating frequency of the piezoelectric transformer solely by
evaluating the phase angle over the frequency.
[0016] In one possible configuration, a phase detector in the
feedback path is used to compare a signal of the input voltage with
a signal proportional to the input current on the input side of the
piezoelectric transformer, and from this an output signal of the
phase detector is ascertained. This output signal of the phase
detector is proportional to the absolute value of the phase angle
between the input voltage and the input current and serves as phase
information for the input impedance of the piezoelectric
transformer. Such measures allow the phase offset of these two
electrical variables to be inferred using simple means on the basis
of a comparison of input voltage and input current.
[0017] In another configuration, an impedance analyzer in the
feedback path is used to sample a signal proportional to the input
current on the input side of the piezoelectric transformer. A
Fourier transformation is used to compute the phase angle of the
signal from the samples, which phase angle finally serves as phase
information for the input impedance of the piezoelectric
transformer. In this manner too, it is possible for the phase angle
for regulating the optimum operating frequency to be
ascertained.
[0018] Advantageously, the method of the type explained involves
the piezoelectric transformer being operated as a plasma generator
such that an input voltage at a frequency regulated in accordance
with the method, on the input side, is converted into an output
voltage, as a result of which a plasma is produced on the output
side on account of ionization of an operating gas flowing around
the plasma generator. The operating gas may be air or else a noble
gas (e.g., argon), for example.
[0019] Particularly when the piezoelectric transformer is
accordingly operated as a plasma generator, a frequency at which
the plasma generator has a maximum efficiency is dependent on
numerous parameters, inter alia also on the operating environment
used (operating gas, temperature, etc.). The explained method for
frequency regulation can be used to adapt the operation of the
plasma generator, particularly its operating frequency, to suit
different operating environments and operating conditions. On
account of the purely input-side capture of the required
information for the frequency regulation, the operating response of
the plasma generator on its output side, and hence the plasma
production, is not adversely influenced. Nevertheless, the plasma
generator can be operated under always optimum conditions. As a
result, heating of the piezoelectric element or component is also
reduced to a minimum. Moreover, the plasma generator can also be
operated at higher plasma powers.
[0020] A further advantage of the application of the explained
method for frequency regulation during operation of a piezoelectric
plasma generator is that it is possible to react to an operating
error (e.g., ignition against conductive items, touching, etc.).
The reason is that such situations result in the phase or the phase
angle of the input impedance being greatly altered, this being able
to be recognized by the method explained. As a result, actuation
can reduce the input power, for example.
[0021] In a further aspect, the aforementioned object is achieved
by a circuit arrangement according to claim 6. The circuit
arrangement comprises: a piezoelectric transformer having an input
side and an output side, an AC voltage source for producing an
input voltage of predetermined frequency on the input side of the
piezoelectric transformer, a detection apparatus that is set up in
a feedback path between the input side of the piezoelectric
transformer and the AC voltage source to capture a phase
information for the input impedance of the piezoelectric
transformer, and a regulating apparatus that is set up to evaluate
the captured phase information in respect of a predetermined phase
criterion and to prescribe a frequency for producing the input
voltage to the AC voltage source on the basis of the evaluated
phase information.
[0022] In comparison with conventional regulating arrangements for
frequency regulation of a piezoelectric transformer, such a circuit
arrangement has the advantage that frequency regulation of the
piezoelectric transformer can be performed solely by an information
captured on the input side of the transformer. As already explained
in connection with the method above, the frequency regulation is
effected on the basis of a captured phase information for the input
impedance of the piezoelectric transformer. This allows a simple
design of the circuit arrangement and nevertheless optimum
regulation of the transformer in respect of an operating frequency
at maximum efficiency. Further, the circuit arrangement does not
adversely influence the operation of the transformer because the
information needed for regulation is tapped off only on the input
side of the piezoelectric transformer.
[0023] In one possible embodiment, the detection apparatus
comprises a phase detector that is connected up such that the
signal of the input voltage is applied to a first input of the
phase detector and a signal proportional to the input current on
the input side of the piezoelectric transformer is applied to a
second input of the phase detector. In this case, the phase
detector is set up to output an output signal that comprises the
phase information of the input impedance at an output. Such a phase
detector allows the phase information of the input impedance to be
captured in a simple manner.
[0024] In accordance with an alternative embodiment, the detection
apparatus comprises an impedance analyzer that is connected up such
that a signal proportional to the input current on the input side
of the piezoelectric transformer is applied to the input of said
impedance analyzer. The impedance analyzer is set up to sample the
signal and to supply the samples to a Fourier transformation to
compute the phase angle of the signal. This phase angle corresponds
to the phase angle of the input impedance of the piezoelectric
transformer.
[0025] Advantageously, the piezoelectric transformer in the circuit
arrangement of the type explained is a piezoelectric plasma
generator that is set up to produce an output high voltage on the
output side from the input voltage on the input side, so that a
plasma is produced on the output side on account of ionization of
an operating gas flowing around the plasma generator.
[0026] Further advantageous embodiments are disclosed in the
subclaims and in the description of the figures that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The invention is explained in more detail below on the basis
of exemplary embodiments with the aid of multiple figures, in
which:
[0028] FIG. 1 shows different characteristics for the absolute
value and also the phase angle of the input impedance of a
piezoelectric plasma generator over frequency;
[0029] FIG. 2 shows different characteristics for the efficiency of
the piezoelectric plasma generator in connection with FIG. 1 over
frequency;
[0030] FIG. 3A shows a circuit arrangement for frequency regulation
for a piezoelectric plasma generator in accordance with an
embodiment;
[0031] FIG. 3B shows the design of a detection apparatus in a
circuit arrangement as shown in FIG. 3A; and
[0032] FIG. 4 shows a circuit arrangement for frequency regulation
for a piezoelectric plasma generator in accordance with a further
embodiment.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0033] FIG. 1 shows different characteristics Z1, Z2, Z3 for the
absolute value of the input impedance ("|Zin|") in ohms (.OMEGA.)
of a piezoelectric plasma generator and also different
characteristics P1, P2, P3 for the corresponding phase angle phi of
the input impedance (".phi.(Zin)") in degrees)(.degree.) over
frequency in kHz. The explanations that follow relate to actuation
of the piezoelectric plasma generator at anti-resonance. The
underlying principle is also valid for plasma generators that are
designed for operation at resonance, however.
[0034] FIG. 1 shows different operating responses from an
accordingly operated plasma generator. The characteristics Zi and
P1 represent a first operating response. In this case, the plasma
generator is excited with a low input voltage, so that the output
voltage is not high enough to generate a plasma. In this case, the
plasma generator can be described as a piezoelectric transformer
during no-load operation. The efficiency in this case is highest at
a phase angle of 0.degree. and at the same time at a maximum of the
impedance. This is illustrated in FIG. 1 by the fact that the
characteristic curve P1 comprises a zero crossing at the frequency
F1 (see vertical marker), at which there is a maximum for the
absolute value of the impedance Z1. FIG. 2 shows a local maximum at
the corresponding frequency (see vertical marker Fl) in the
characteristic of the efficiency W1.
[0035] If the input voltage is increased, then the plasma generator
begins to generate plasma. This response can be described
approximately as a voltage-dependent load resistance at the output
of the piezoelectric plasma generator. This operating response is
represented in FIG. 1 by the characteristic curves P2 and Z2. In
contrast to the response of the characteristic curves P1 and Z1,
the efficiency in this operating response is no longer highest at a
maximum of the impedance (see, e.g., local maximum of the
characteristic curve Z2 in FIG. 1), but rather at a phase angle of
0.degree. (see, e.g., zero of the characteristic curve P2 at a
correspondingly lower frequency F2). FIG. 2 shows this response, in
the case of which the characteristic of the efficiency W2 has a
local maximum at F2.
[0036] If the voltage at the input of the plasma generator is
increased further, then there is a point at which the inductive
response of the component disappears completely and the phase angle
is always less than 0.degree.. See the shape of the characteristic
curves P3 and Z3 in FIG. 1. In this case, the maximum efficiency is
at a maximum of the phase angle P3 at the frequency F3. In this
regard, see, e.g., also the local maximum of the efficiency
characteristic W3 in FIG. 2 at the frequency F3.
[0037] From the examinations explained above, as evidenced by FIGS.
1 and 2, it follows that it is possible, solely on the basis of the
frequency-dependent characteristic of the phase angle, to operate
the plasma generator at maximum efficiency each time under
arbitrary external conditions.
[0038] This insight can be exploited for frequency regulation of
the plasma generator.
[0039] FIG. 3A shows an embodiment of a circuit arrangement for
frequency regulation for a corresponding plasma generator. In
particular, the circuit arrangement comprises a piezoelectric
transformer 1 that operates as a piezoelectric plasma generator.
The piezoelectric transformer 1 comprises an input side 1a and an
output side 1b. Connected on the input side is an AC voltage source
2 that produces a sinusoidal AC voltage to excite the piezoelectric
transformer 1 on the input side 1a thereof. The AC voltage source 2
may be a DDS (direct digital synthesis) sine generator, for
example. However, it is also possible for the AC voltage source 2
to be embodied as an analog voltage controlled oscillator
(VCO).
[0040] The AC voltage produced in this way is preamplified by means
of a power amplifier 3, so that an input voltage signal u is
applied on the input side 1a of the piezoelectric transformer 1.
This input voltage u can be used to excite the piezoelectric
transformer 1 into mechanical oscillation, so that the output side
1b thereof produces an output high voltage for plasma production
for an operating gas flowing around the piezoelectric transformer
1, e.g., air.
[0041] Further, a detection apparatus 4 is set up in a feedback
path of the circuit arrangement shown in FIG. 3A, to which
detection apparatus both the input voltage signal u and a signal
u(i) are supplied. The latter is a voltage signal proportional to
the input current i on the input side 1a of the transformer 1. The
signal u(i) is obtained via a current shunt 6. The two signals u
and u(i) are processed in the detection apparatus 4 to produce an
output signal u=f(|.phi.|) (voltage signal as a function of the
absolute value of the phase angle of the input impedance of the
transformer 1). In this embodiment, the output signal is
proportional to the absolute value of the phase angle. In other
words, the output signal of the detection apparatus 4 represents a
phase information, specifically the phase offset (phase angle)
between the time-dependent input voltage u and the time-dependent
input current (represented by the signal u(i)) on the input side 1a
of the transformer 1.
[0042] The output signal of the detection apparatus 4 is
additionally supplied to a regulating apparatus 5 that evaluates
the output signal. On the basis of this evaluation, a new frequency
(or a frequency altered by a particular absolute value) is possibly
computed that is supplied to the AC voltage source 2 as an
actuating signal. The regulating apparatus 5 may be set up as a
microcontroller, for example. In particular, the regulating
apparatus 5 evaluates the phase information that is captured by
means of the detection apparatus 4 and output as an output signal
to determine whether or not the captured phase angle of the input
impedance of the transformer 1 satisfies a predetermined phase
criterion. In this context, the regulating apparatus 5
advantageously evaluates the adequate satisfaction or reaching of a
zero crossing (zero) or of a local extreme of the phase angle. As
explained with regard to FIG. 1 and FIG. 2, this is because the
maximum efficiencies of the transformer 1 that can be attained are
located at these points of the characteristics of the applicable
phase angle over frequency in the respective operating
situations.
[0043] It is conceivable for regulation to be performed starting
from a particular frequency such that the frequency is changed
continuously and the captured phase angle of the input impedance of
the transformer is evaluated in accordance with the explained
measures in respect of the approach toward and finally adequate
satisfaction of the predetermined phase criterion. This regulation
can be effected during continuous operation of the piezoelectric
transformer 1. Alternatively, it is also conceivable for a
predetermined frequency band to be initially swept for a specific
operating situation (e.g., by means of a sweep or chirp signal),
and for the phase angle to be evaluated in respect of the desired
phase criterion and to be subsequently regulated to the suitable
frequency by which the phase criterion is adequately satisfied.
Thereafter, the piezoelectric transformer 1 can be operated at this
frequency in optimum fashion. In this case, it is additionally also
possible for further readjustment to be effected during operation,
e.g., if particular operating parameters such as operating gas
volume, temperature, etc. change.
[0044] If a captured phase information satisfies an explained phase
criterion adequately (e.g., in a predetermined small range around
the defined phase criterion), then the transformer 1 shown in FIG.
3A can be operated at an optimum efficiency at the corresponding
frequency. The circuit arrangement shown in FIG. 3A therefore has
the advantage that a captured phase information for the input
impedance of the transformer 1 can be used to regulate to an
optimum operating frequency at which a predetermined phase
criterion (see, e.g., FIGS. 1 and 2) is satisfied.
[0045] FIG. 3B shows the detection apparatus 4 shown in FIG. 3A in
a detail view. In this case, the detection apparatus 4 is embodied
as a phase detector. First, the input voltage signal u and the
signal u(i), which is proportional to the input current, are
supplied to a respective comparator K1 and K2 to determine the zero
crossing of the respective signals. Subsequently, the signals
processed in this way are applied to the inputs of an XOR
(Exclusive OR) gate 7. The XOR gate 7 outputs a high level at its
output so long as the signals u and u(i) have opposite arithmetic
signs. Conversely, the XOR gate 7 outputs a low level so long as
the signals u and u(i) have the same arithmetic sign. The output
signal of the XOR gate 7 is further forwarded to a low-pass filter
TP that averages the signal of the XOR gate 7. The output signal
u=f(|.phi.|) (voltage signal as a function of the absolute value of
the phase angle of the input impedance of the transformer i)
obtained in this way is zero if the signals u and u(i) are in
phase. If the signals have a phase shift of +180 degrees or -180
degrees, then the output signal is at a maximum.
[0046] In this manner, although the phase detector 4 embodied in
this way cannot distinguish between positive and negative phases, a
signal (phase signal) is obtained that is proportional to the
absolute value of the phase angle. This signal can be transferred
to the regulating apparatus 5 (see, e.g., FIG. 3A), in which an
appropriate algorithm is used to ascertain the approach toward and
reaching of the desired phase criterion that finally leads to the
optimum operating frequency by iteratively changing the frequency
and evaluating the phase signal. An appropriate algorithm can
comprise a zero method and/or an extreme value search algorithm,
for example. By way of example, it is also conceivable to use LQ
regulating methods for the optimized finding of an optimum phase
criterion (as a Q factor functional) to determine the appropriate
operating frequency. In this case, all kinds of regulating
algorithms or even combinations of corresponding methods are
conceivable.
[0047] FIG. 4 shows an alternative embodiment of a circuit
arrangement for frequency regulation for a piezoelectric
transformer 1. Some essential components of the circuit arrangement
correspond to those of the circuit arrangement shown in FIG. 3A.
The only difference in the circuit arrangement shown in FIG. 4 is
that the detection apparatus 4 and the AC voltage source 2 are
structurally combined in one module and the detection apparatus 4
is set up as an impedance analyzer. Therefore, the detection
apparatus 4 in the feedback path of the circuit arrangement merely
has a voltage signal u(i) transferred to it that, analogously to
FIG. 3A, is formed by means of a current shunt 6 and is
proportional to the input current on the input side is of the
piezoelectric transformer 1. The signal u(i) is oversampled in the
impedance analyzer 4 and the applicable phase angle of the signal
is determined from the samples by Fourier transformation. This can
involve an algorithmic method of a fast Fourier transformation
(FFT) being applied, for example. An appropriate output signal
u=f(.phi.) can then be transferred to the regulating apparatus 5
and evaluated therein conveniently. The regulating apparatus 5 can
adjust the frequency as a manipulated variable for the AC voltage
source 2 using software, for example. Evaluation of the phase angle
using an impedance analyzer 4 as shown in FIG. 4 has the advantage
that the arithmetic sign of the phase angle can also be taken into
consideration and incorporated into an evaluation.
[0048] The depicted embodiments are chosen merely by way of
example. The regulating method explained herein and the circuit
arrangement explained allow frequency regulation for a
piezoelectric transformer, particularly a piezoelectric plasma
generator, to set an optimum operating frequency in respective
operating situations such that the piezoelectric transformer can be
operated at an optimum efficiency. The advantage of the method and
of the circuit arrangement is that an appropriate regulatory
information is obtained merely from signals that can be tapped off
on an input side of the transformer. In this manner, tapping-off
and feedback of signals on an output side of the transformer, as a
result of which the operation of the transformer would be adversely
influenced, are dispensed with. Further, a corresponding circuit
arrangement allows a simple design.
* * * * *